SENSOR ELEMENT OF NOx SENSOR

ABSTRACT

A sensor element includes: a base part containing an oxygen-ion conductive solid electrolyte as a constituent material; at least one internal space into which a measurement gas is introduced; and at least one pump cell including an internal electrode disposed to face the internal space, an out-of-space pump electrode disposed at a location other than the internal space, and a portion of the base part located between these electrodes, the internal electrode includes a noble metal, the solid electrolyte, and a pore, and, in the internal electrode, a degree of variation of a boundary of a first region formed of the base part or the solid electrolyte contiguous with the base part and a second region occupied by the noble metal and the pore in a thickness direction of the element is 0.5 μm or more and 6.5 μm or less.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese applicationsJP2021-008725, filed on Jan. 22, 2021 and JP2022-002293, filed on Jan.11, 2022, the contents of which is hereby incorporated by reference intothis application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a sensor element of a limiting currentNOx sensor, and, in particular, to a configuration of a measurementelectrode of the sensor element.

Description of the Background Art

As a device for measuring the concentration of NOx in a measurement gas,such as a combustion gas and an exhaust gas from an internal combustionengine typified by an engine of a vehicle, a NOx sensor including asensor element including a base formed of oxygen-ion conductive solidelectrolyte ceramics, such as zirconia (ZrO₂), has been known (seeJapanese Patent No. 2885336 and WO 2019/188613, for example).

The sensor element (a NOx sensor element) of the NOx sensor includesvarious electrodes (e.g., a pump electrode, a measurement electrode, anda reference electrode). These electrodes are porous cermet electrodeseach formed of a composite material of a noble metal as a catalyst andzirconia as an electrolyte, and having a porous structure including manypores (cavities). As the catalytic noble metal, Pt and Pt with traceamounts of other substances (e.g., Rh and other noble metals) added areused. The NOx sensor element utilizes, at operation thereof, a catalyticreaction of the catalytic noble metal used for the electrodes andoxygen-ion conductivity of zirconia used for the base, and is thus usedin a state of being heated to a relatively high sensor element drivingtemperature (600° C. to 900° C.).

The measurement gas, such as an exhaust gas from an engine, typicallycontains O₂. Thus, in a NOx sensor as disclosed in Japanese Patent No.2885336 and WO 2019/188613, 02 in a measurement gas introduced into aNOx sensor element is first removed (separated from NOx) by anelectrochemical pump cell of the NOx sensor element, NOx is thendecomposed into O₂ and N₂ using a catalytic reaction of a measurementelectrode, and a NOx concentration of the measurement gas is measuredbased on the magnitude of an oxygen-ion current flowing through ameasurement pump cell in the decomposition.

When the measurement electrode is exposed to O₂, Pt and Pt/Rh of theelectrode react with O₂ to generate oxides such as PtO, PtO₂, and RhO₂.When noble metals of the electrode decrease due to generation of theoxides, catalytic reactivity of the measurement electrode is reduced.These oxides have lower vapor pressure than Pt, and thus are likely tovaporize at a lower temperature than Pt. The vaporization leads toreduction in number of three-phase interfaces of a catalyst, anelectrolyte, and a pore and two-phase interfaces of the catalyst and theelectrolyte in the measurement electrode, and thus also causes reductionin catalytic reactivity of the measurement electrode.

As described above, oxygen is removed in advance from the measurementgas reaching the measurement electrode, but a small amount of oxygenstill remains in the measurement gas. Furthermore, O₂ generated bydecomposition of NOx might remain near the measurement electrode withoutpromptly being ionized.

Thus, by continued use of the NOx sensor, NOx sensitivity is graduallyreduced due to reduction in catalytic reactivity of the measurementelectrode.

Elaborating on mass transfer near the measurement electrode atmeasurement, O₂ and NOx in the measurement gas having reached themeasurement electrode first transfer through pores of the measurementelectrode (referred to as a mass transfer process). When O₂ and NOx comeinto contact with a catalytic metal within the pores, Pt as a maincomponent of the catalytic metal decomposes NO into N₂ and O₂, andfurther strips off an electron from O₂ to generate an oxygen ion. Thegenerated oxygen ion is taken, at a two-phase interface of the catalystand the electrolyte, from Pt into zirconia as the solid electrolyte, andtransfers within zirconia due to a potential difference caused betweenthe measurement electrode and an external electrode by application of avoltage by an external power supply (referred to as a charge transferprocess).

If there are an oxygen ion not taken from the catalytic metal intozirconia and, further, oxygen before being ionized (generically referredto as surplus oxygen) in this process, however, the catalytic metalincluding Pt as the main component is oxidized by the surplus oxygeninside or at the surface of the electrode to generate PtO, PtO₂, and thelike. These oxides gradually vaporize.

The inventors of the present invention have reasoned that reduction ofthe surplus oxygen not taken into zirconia can suppress vaporization ofthe oxides from the measurement electrode, and suppress reduction insensitivity of the NOx sensor, and have conceived of the presentinvention through intense study.

WO 2019/188613 notes that, at oxidation of Pt in the measurementelectrode, impurities present in the electrode further reduce atemperature at which the oxides vaporize, and discloses thatvaporization of the oxides is suppressed by providing a gettering layergettering such impurities. WO 2019/188613, however, neither disclosesnor suggests suppression of vaporization of the oxides by reduction ofthe surplus oxygen.

SUMMARY

The present invention relates to a sensor element of a limiting currenttype NOx sensor, and is, in particular, directed to a configuration of ameasurement electrode of the sensor element.

According to the present invention, a sensor element of a limitingcurrent type NOx sensor includes: base part containing an oxygen-ionconductive solid electrolyte as a constituent material; at least oneinternal space into which a measurement gas is introduced; and at leastone pump cell including an internal electrode disposed to face the atleast one internal space, an out-of-space pump electrode disposed at alocation other than the at least one internal space, and a portion ofthe base part located between the internal electrode and theout-of-space pump electrode, wherein the internal electrode includes anoble metal, the solid electrolyte, and a pore, and, in the internalelectrode, a region boundary variation degree being a degree ofvariation of a boundary of a first region and a second region in athickness direction of the element is 0.5 μm or more and 6.5 μm or less,the first region being formed of the base part or the solid electrolytecontiguous with the base part, the second region being occupied by thenoble metal and the pore.

Deterioration of the measurement electrode due to the presence ofsurplus oxygen not taken into the measurement electrode is therebysuitably suppressed. A NOx sensor in which deterioration of measurementsensitivity is suitably suppressed even if the NOx sensor is usedcontinuously is thereby achieved.

It is thus an object of the present invention to provide a NOx sensor inwhich reduction in sensitivity due to oxidation of a measurementelectrode is suppressed even if the NOx sensor is used continuously.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing one example of a configurationof a gas sensor 100;

FIG. 2 is a model diagram showing a partial cross section of ameasurement electrode 44 along a thickness direction of an element;

FIG. 3 is a diagram for describing a way to evaluate a region boundaryvariation degree being a degree of variation of a boundary of a firstregion and a second region in the thickness direction of the element;

FIG. 4 is a model diagram of a partial cross section of a measurementelectrode 44Z having a configuration in which three phases coexist inthe electrode as a whole;

FIG. 5 is a flowchart of processing at manufacture of a sensor element101;

FIG. 6 is a diagram showing procedures for forming a pattern toeventually be the measurement electrode 44; and

FIG. 7 is a graph showing results of a continuous drive test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<General Configuration of Gas Sensor>

FIG. 1 is a diagram schematically showing one example of a configurationof a gas sensor 100 according to an embodiment. The gas sensor 100 is alimiting current type NOx sensor sensing NOx and measuring theconcentration thereof using a sensor element 101. The gas sensor 100further includes a controller 110 controlling operation of each part andidentifying the NOx concentration based on a NOx current flowing throughthe sensor element 101. FIG. 1 includes a vertical cross-sectional viewtaken along a longitudinal direction of the sensor element 101.

The sensor element 101 is a planar (elongated planar) element having astructure in which six solid electrolyte layers, namely, a firstsubstrate layer 1, a second substrate layer 2, a third substrate layer3, a first solid electrolyte layer 4, a spacer layer 5, and a secondsolid electrolyte layer 6 each formed of zirconia (ZrO₂) (e.g., yttriastabilized zirconia (YSZ)) as an oxygen-ion conductive solid electrolyteare laminated in the stated order from a bottom side of FIG. 1. Thesolid electrolyte forming these six layers is dense and airtight. Asurface on an upper side and a surface on a lower side of each of thesesix layers in FIG. 1 are hereinafter also simply referred to as an uppersurface and a lower surface, respectively. A part of the sensor element101 formed of the solid electrolyte as a whole is generically referredto as a base part.

The sensor element 101 is manufactured, for example, by performingpredetermined processing, printing of circuit patterns, and the like onceramic green sheets corresponding to the respective layers, thenlaminating them, and further firing them for integration.

Between a lower surface of the second solid electrolyte layer 6 and anupper surface of the first solid electrolyte layer 4 in one leading endportion of the sensor element 101, a first diffusion control part 11doubling as a gas inlet 10, a buffer space 12, a second diffusioncontrol part 13, a first internal space 20, a third diffusion controlpart 30, a second internal space 40, a fourth diffusion control part 60,and a third internal space 61 are formed adjacent to each other tocommunicate in the stated order.

The buffer space 12, the first internal space 20, the second internalspace 40, and the third internal space 61 are spaces (regions) insidethe sensor element 101 looking as if they were provided by hollowing outthe spacer layer 5, and having an upper portion, a lower portion, and aside portion respectively defined by the lower surface of the secondsolid electrolyte layer 6, the upper surface of the first solidelectrolyte layer 4, and a side surface of the spacer layer 5. The gasinlet 10 may similarly look as if it was provided by hollowing out thespacer layer 5 at a leading end surface (at the left end in FIG. 1) ofthe sensor element 101 separately from the first diffusion control part11. In this case, the first diffusion control part 11 is formed insideand adjacent to the gas inlet 10.

The first diffusion control part 11, the second diffusion control part13, the third diffusion control part 30, and the fourth diffusioncontrol part 60 are each provided as two horizontally long slits (whoseopenings have longitudinal directions perpendicular to the page of FIG.1). A part extending from the gas inlet 10 to the third internal space61 is also referred to as a gas distribution part.

At a location farther from the leading end than the gas distributionpart is, a reference gas introduction space 43 having a side portiondefined by a side surface of the first solid electrolyte layer 4 isprovided between an upper surface of the third substrate layer 3 and alower surface of the spacer layer 5. For example, air is introduced intothe reference gas introduction space 43 as a reference gas atmeasurement of the NOx concentration.

An air introduction layer 48 is a layer formed of porous alumina, andthe reference gas is introduced into the air introduction layer 48through the reference gas introduction space 43. The air introductionlayer 48 is formed to cover a reference electrode 42.

The reference electrode 42 is an electrode formed to be sandwichedbetween the upper surface of the third substrate layer 3 and the firstsolid electrolyte layer 4, and the air introduction layer 48 leading tothe reference gas introduction space 43 is provided around the referenceelectrode 42 as described above. As will be described below, an oxygenconcentration (oxygen partial pressure) in the first internal space 20and the second internal space 40 can be measured using the referenceelectrode 42.

In the gas distribution part, the gas inlet 10 (first diffusion controlpart 11) is a part opening to an external space, and a measurement gasis taken from the external space into the sensor element 101 through thegas inlet 10.

The first diffusion control part 11 is a part providing predetermineddiffusion resistance to the taken measurement gas.

The buffer space 12 is a space provided to guide the measurement gasintroduced through the first diffusion control part 11 to the seconddiffusion control part 13.

The second diffusion control part 13 is a part providing predetermineddiffusion resistance to the measurement gas introduced from the bufferspace 12 into the first internal space 20.

In introducing the measurement gas from outside the sensor element 101into the first internal space 20, the measurement gas having abruptlybeen taken into the sensor element 101 through the gas inlet 10 due topressure fluctuations (pulsation of exhaust pressure in a case where themeasurement gas is an exhaust gas of a vehicle) of the measurement gasin the external space is not directly introduced into the first internalspace 20 but is introduced into the first internal space 20 afterconcentration fluctuations of the measurement gas are canceled throughthe first diffusion control part 11, the buffer space 12, and the seconddiffusion control part 13. This makes the concentration fluctuations ofthe measurement gas introduced into the first internal space 20 almostnegligible.

The first internal space 20 is provided as a space to adjust oxygenpartial pressure of the measurement gas introduced through the seconddiffusion control part 13. The oxygen partial pressure is adjusted byoperation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell including an innerpump electrode 22, an outer (out-of-space) pump electrode 23, and thesecond solid electrolyte layer 6 sandwiched between these electrodes.The inner pump electrode 22 has a ceiling electrode portion 22 aprovided on substantially the entire lower surface of a portion of thesecond solid electrolyte layer 6 facing the first internal space 20, andthe outer pump electrode 23 is provided in a region, on an upper surfaceof the second solid electrolyte layer 6 (one main surface of the sensorelement 101), corresponding to the ceiling electrode portion 22 a to beexposed to the external space.

The inner pump electrode 22 is formed on upper and lower solidelectrolyte layers (the second solid electrolyte layer 6 and the firstsolid electrolyte layer 4) defining the first internal space 20.Specifically, the ceiling electrode portion 22 a is formed on the lowersurface of the second solid electrolyte layer 6, which provides aceiling surface to the first internal space 20, and a bottom electrodeportion 22 b is formed on the upper surface of the first solidelectrolyte layer 4, which provides a bottom surface to the firstinternal space 20. The ceiling electrode portion 22 a and the bottomelectrode portion 22 b are connected by a conducting portion (notillustrated) provided on a side wall surface (an inner surface) of thespacer layer 5 forming opposite side wall portions of the first internalspace 20.

The ceiling electrode portion 22 a and the bottom electrode portion 22 bare provided to be rectangular in plan view. Only the ceiling electrodeportion 22 a or only the bottom electrode portion 22 b may be provided.

The inner pump electrode 22 and the outer pump electrode 23 are eachformed as a porous cermet electrode. In particular, the inner pumpelectrode 22 to be in contact with the measurement gas is formed using amaterial having a weakened reducing ability with respect to a NOxcomponent in the measurement gas. For example, the inner pump electrode22 is formed as a cermet electrode of an Au—Pt alloy containing Au ofapproximately 0.6 wt % to 1.4 wt % and ZrO₂ to have a porosity of 5% to40% and a thickness of 5 μm to 20 μm. A weight ratio Pt:ZrO₂ of theAu—Pt alloy and ZrO₂ is only required to be approximately 7.0:3.0 to5.0:5.0.

On the other hand, the outer pump electrode 23 is formed, for example,as a cermet electrode of Pt or an alloy thereof and ZrO₂ to berectangular in plan view.

The main pump cell 21 can pump out oxygen in the first internal space 20to the external space or pump in oxygen in the external space to thefirst internal space 20 by applying, from a variable power supply 24, adesired pump voltage Vp0 between the inner pump electrode 22 and theouter pump electrode 23 to allow a main pump current Ip0 to flow betweenthe inner pump electrode 22 and the outer pump electrode 23 in apositive or negative direction. The pump voltage Vp0 applied between theinner pump electrode 22 and the outer pump electrode 23 in the main pumpcell 21 is also referred to as a main pump voltage Vp0.

To detect the oxygen concentration (oxygen partial pressure) in anatmosphere in the first internal space 20, the inner pump electrode 22,the second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, and thereference electrode 42 constitute a main sensor cell 80 as anelectrochemical sensor cell.

The oxygen concentration (oxygen partial pressure) in the first internalspace 20 can be known by measuring electromotive force V0 in the mainsensor cell 80.

Furthermore, the controller 110 performs feedback control of the mainpump voltage Vp0 so that the electromotive force V0 is constant, therebyto control the main pump current Ip0. The oxygen concentration in thefirst internal space 20 is thereby maintained at a predeterminedconstant value.

The third diffusion control part 30 is a part providing predetermineddiffusion resistance to the measurement gas having an oxygenconcentration (oxygen partial pressure) controlled by operation of themain pump cell 21 in the first internal space 20, and guiding themeasurement gas to the second internal space 40.

The second internal space 40 is provided as a space to further adjustthe oxygen partial pressure of the measurement gas introduced throughthe third diffusion control part 30. The oxygen partial pressure isadjusted by operation of an auxiliary pump cell 50. The oxygenconcentration of the measurement gas is adjusted with higher accuracy inthe second internal space 40.

After the oxygen concentration (oxygen partial pressure) is adjusted inadvance in the first internal space 20, the auxiliary pump cell 50further adjusts the oxygen partial pressure of the measurement gasintroduced through the third diffusion control part 30 in the secondinternal space 40.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cellincluding an auxiliary pump electrode 51, the outer pump electrode 23(not limited to the outer pump electrode 23 and only required to be anyappropriate electrode outside the sensor element 101), and the secondsolid electrolyte layer 6. The auxiliary pump electrode 51 has a ceilingelectrode portion 51 a provided on substantially the entire lowersurface of a portion of the second solid electrolyte layer 6 facing thesecond internal space 40.

The auxiliary pump electrode 51 is provided in the second internal space40 in a similar form to the inner pump electrode 22 provided in thefirst internal space 20 described previously. That is to say, theceiling electrode portion 51 a is formed on the second solid electrolytelayer 6, which provides a ceiling surface to the second internal space40, and a bottom electrode portion 51 b is formed on the first solidelectrolyte layer 4, which provides a bottom surface to the secondinternal space 40. The ceiling electrode portion 51 a and the bottomelectrode portion 51 b are rectangular in plan view, and are connectedby a conducting portion (not illustrated) provided on the side wallsurface (inner surface) of the spacer layer 5 forming opposite side wallportions of the second internal space 40.

As with the inner pump electrode 22, the auxiliary pump electrode 51 isformed using a material having a weakened reducing ability with respectto the NOx component in the measurement gas.

The auxiliary pump cell 50 can pump out oxygen in an atmosphere in thesecond internal space 40 to the external space or pump in oxygen in theexternal space to the second internal space 40 by applying a desiredvoltage (an auxiliary pump voltage) Vp1 between the auxiliary pumpelectrode 51 and the outer pump electrode 23 under control performed bythe controller 110.

To control the oxygen partial pressure in the atmosphere in the secondinternal space 40, the auxiliary pump electrode 51, the referenceelectrode 42, the second solid electrolyte layer 6, the spacer layer 5,the first solid electrolyte layer 4, and the third substrate layer 3constitute an auxiliary sensor cell 81 as an electrochemical sensorcell.

The auxiliary pump cell 50 performs pumping using a variable powersupply 52 whose voltage is controlled based on electromotive force V1detected by the auxiliary sensor cell 81 in accordance with the oxygenpartial pressure in the second internal space 40. The oxygen partialpressure in the atmosphere in the second internal space 40 is therebycontrolled to a low partial pressure having substantially no effect onmeasurement of NOx.

At the same time, a resulting auxiliary pump current Ip1 is used tocontrol the electromotive force in the main sensor cell 80.Specifically, the auxiliary pump current Ip1 is input, as a controlsignal, into the main sensor cell 80, and, through control of theelectromotive force V0 therein, the oxygen partial pressure of themeasurement gas introduced through the third diffusion control part 30into the second internal space 40 is controlled to have a gradient thatis always constant. In use as the NOx sensor, the oxygen concentrationin the second internal space 40 is maintained at a constant value ofapproximately 0.001 ppm by the action of the main pump cell 21 and theauxiliary pump cell 50.

The fourth diffusion control part 60 is a part providing predetermineddiffusion resistance to the measurement gas having an oxygenconcentration (oxygen partial pressure) controlled by operation of theauxiliary pump cell 50 in the second internal space 40, and guiding themeasurement gas to the third internal space 61.

The third internal space 61 is provided as a space to perform processingconcerning measurement of the nitrogen oxide (NOx) concentration of themeasurement gas introduced through the fourth diffusion control part 60.The NOx concentration is measured by operation of a measurement pumpcell 41 in the third internal space 61. The measurement gas having theoxygen concentration adjusted with high accuracy in the second internalspace 40 is introduced into the third internal space 61, so that the NOxconcentration can be measured with high accuracy in the gas sensor 100.

The measurement pump cell 41 measures the NOx concentration of themeasurement gas in the third internal space 61. The measurement pumpcell 41 is an electrochemical pump cell including a measurementelectrode 44, the outer pump electrode 23, the second solid electrolytelayer 6, the spacer layer 5, and the first solid electrolyte layer 4.The measurement electrode 44 is provided on an upper surface of aportion of the first solid electrolyte layer 4 facing the third internalspace 61 to be separated from the third diffusion control part 30.

The measurement electrode 44 is a porous cermet electrode of a noblemetal and a solid electrolyte. For example, the measurement electrode 44is formed as a cermet electrode of Pt or an alloy of Pt and anothernoble metal, such as Rh, and ZrO₂ as a constituent material for thesensor element 101. The measurement electrode 44 also functions as a NOxreduction catalyst to reduce NOx present in the atmosphere in the thirdinternal space 61.

The measurement pump cell 41 can pump out oxygen generated throughdecomposition of NOx in an atmosphere around the measurement electrode44, and detect the amount of generated oxygen as a pump current Ip2under control performed by the controller 110.

In the present embodiment, the measurement electrode 44 has a two-layerconfiguration including a region where pores are not present and aregion where pores are present. Details thereof will be described below.

To detect the oxygen partial pressure around the measurement electrode44, the second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, the measurementelectrode 44, and the reference electrode 42 constitute a measurementsensor cell 82 as an electrochemical sensor cell. A variable powersupply 46 is controlled based on electromotive force V2 detected by themeasurement sensor cell 82 in accordance with the oxygen partialpressure around the measurement electrode 44.

NOx in the measurement gas introduced into the third internal space 61is reduced by the measurement electrode 44 (2NO→N₂+O₂) to generateoxygen. Oxygen as generated is to be pumped by the measurement pump cell41, and, in this case, a voltage (measurement pump voltage) Vp2 of thevariable power supply 46 is controlled so that the electromotive forceV2 detected by the measurement sensor cell 82 is constant. The amount ofoxygen generated around the measurement electrode 44 is proportional tothe NOx concentration of the measurement gas, and thus the NOxconcentration of the measurement gas is to be calculated using the pumpcurrent Ip2 in the measurement pump cell 41. The pump current Ip2 ishereinafter also referred to as a NOx current Ip2.

In the case that the measurement electrode 44, the first solidelectrolyte layer 4, the third substrate layer 3, and the referenceelectrode 42 are combined to constitute an oxygen partial pressuredetection means as an electrochemical sensor cell, electromotive forcein accordance with a difference between the amount of oxygen generatedthrough reduction of a NOx component in the atmosphere around themeasurement electrode 44 and the amount of oxygen contained in referenceair can be detected, and the concentration of the NOx component in themeasurement gas can thereby be determined.

The second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, the outer pumpelectrode 23, and the reference electrode 42 constitute anelectrochemical sensor cell 83, and oxygen partial pressure of themeasurement gas outside the sensor can be detected using electromotiveforce Vref obtained by the sensor cell 83.

The sensor element 101 further includes a heater part 70 playing a rolein temperature adjustment of heating the sensor element 101 andmaintaining the temperature thereof to enhance oxygen ion conductivityof the solid electrolyte forming the base part.

The heater part 70 mainly includes a heater electrode 71, a heaterelement 72, a heater lead 72 a, a through hole 73, a heater insulatinglayer 74, and a heater resistance detection lead, which is notillustrated in FIG. 1. A portion of the heater part 70 other than theheater electrode 71 is buried in the base part of the sensor element101.

The heater electrode 71 is an electrode formed to be in contact with alower surface of the first substrate layer 1 (the other main surface ofthe sensor element 101).

The heater element 72 is a resistive heating element provided betweenthe second substrate layer 2 and the third substrate layer 3. The heaterelement 72 generates heat by being powered from a heater power supply,which is not illustrated in FIG. 1, outside the sensor element 101through the heater electrode 71, the through hole 73, and the heaterlead 72 a, which constitute a current-carrying path. The heater element72 is formed of Pt, or contains Pt as a main component. The heaterelement 72 is buried, in a predetermined range of the sensor element 101in which the gas distribution part is provided, to oppose the gasdistribution part in a thickness direction of the element. The heaterelement 72 is provided to have a thickness of approximately 10 μm to 20μm.

In the sensor element 101, each part of the sensor element 101 can beheated to a predetermined temperature and the temperature can bemaintained by allowing a current to flow through the heater electrode 71to the heater element 72 to thereby cause the heater element 72 togenerate heat. Specifically, the sensor element 101 is heated so thatthe temperature of the solid electrolyte and the electrodes in thevicinity of the gas distribution part is approximately 700° C. to 900°C. The oxygen ion conductivity of the solid electrolyte forming the basepart of the sensor element 101 is enhanced by the heating. A heatingtemperature of the heater element 72 when the gas sensor 100 is in use(when the sensor element 101 is driven) is referred to as a sensorelement driving temperature.

A degree of heat generation (heater temperature) of the heater element72 is grasped by the magnitude of a resistance value (heater resistance)of the heater element 72.

Although not illustrated in FIG. 1, a thermal shock resistant protectivelayer as a single- or multi-porous layer covering the sensor element 101may further be provided outside in a predetermined range on a side ofthe one leading end portion (side of the left end in FIG. 1) of thesensor element 101. The thermal shock resistant protective layer isprovided to prevent cracking of the sensor element 101 due to thermalshock caused by moisture contained in the measurement gas adhering tothe sensor element 101 and condensing when the gas sensor 100 is in use,and prevent poisoning substances coexisting in the measurement gas fromentering into the sensor element 101. A laminar gap (gap layer) may beformed between the sensor element 101 and the thermal shock resistantprotective layer.

<Detailed Configuration of Measurement Electrode>

A configuration of the measurement electrode 44 of the sensor element101 having a configuration as described above will be described in moredetail next. FIG. 2 is a model diagram showing a partial cross sectionof the measurement electrode 44 along the thickness direction of theelement. Dashed lines La and Lb shown in FIG. 2 are added for ease ofunderstanding.

In FIG. 2, a portion below the dashed line La is the first solidelectrolyte layer 4 formed of a solid electrolyte (zirconia), and themeasurement electrode 44 is provided on the first solid electrolytelayer 4. A portion above the measurement electrode 44 is the thirdinternal space 61 (more particularly, a portion of the third internalspace 61 not occupied by the measurement electrode 44, but hereinaftersimply referred to as the third internal space 61 for the sake ofconvenience).

As described above, the measurement electrode 44 is the porous cermetelectrode of the noble metal, such as Pt and an alloy of Pt and Rh andthe like, and the solid electrolyte (zirconia). Thus, as shown in FIG.2, the measurement electrode 44 has a configuration that there coexist aportion formed of a noble metal NM of white in FIG. 2, a portion formedof a solid electrolyte SE of gray (or light gray) in FIG. 2, and aportion formed of a pore (cavity) CV of black (or dark gray) in FIG. 2.As with the solid electrolyte SE, the first solid electrolyte layer 4 isof gray in FIG. 2. This is because they are both formed of zirconia. Thethird internal space 61 is of black in FIG. 2 as with the pore CV, andthis is because they are both spaces or gaps.

The measurement electrode 44 according to the present embodiment,however, does not have a configuration in which the three portions(phases) formed of the noble metal NM, the solid electrolyte SE, and thepore CV randomly coexist as a whole, but has a two-layer configurationincluding a lower layer (also referred to as a two-phase region) 44Lwhere only the noble metal NM and the solid electrolyte SE randomlycoexist, and the pore CV is not present and an upper layer (alsoreferred to as a three-phase region) 44U where the noble metal NM, thesolid electrolyte SE, and the pore CV randomly coexist. A boundarybetween the lower layer 44L and the upper layer 44U is shown in thedashed line Lb in FIG. 2.

Viewed another way, the dashed line La shows a lowermost end of a rangein which the noble metal NM is present in a thickness direction of themeasurement electrode 44, and the dashed line Lb shows a lowermost endof a range in which the pore CV is present in the thickness direction ofthe measurement electrode 44. Expressed yet another way, it can be saidthat the dashed line La is a boundary between a region where the noblemetal NM is present and a region where the noble metal NM is not presentin the thickness direction of the measurement electrode 44, and thedashed line Lb is a boundary between a region where the pore CV ispresent and a region where the pore CV is not present in the thicknessdirection of the measurement electrode 44.

A portion of the solid electrolyte SE forming the measurement electrode44 contiguous with the base part formed of the solid electrolyte of thesensor element 101 (a portion contiguous with the base part beyond thedashed line La) is hereinafter particularly referred to as a contiguousregion RE. Furthermore, a portion of the contiguous region RE belongingto the lower layer 44L (a portion located between the dashed line La andthe dashed line Lb) is referred to as a lower contiguous region REL, anda portion of the contiguous region RE belonging to the upper layer 44U(a portion located between the dashed line Lb and the third internalspace 61) is referred to as an upper contiguous region REU.

As described above, the measurement electrode 44 of the sensor element101 according to the present embodiment is characterized in that themeasurement electrode 44 as a whole is provided as the porous cermetelectrode in which the noble metal NM, the solid electrolyte SE, and thepore CV coexist, but these three phases actually coexist only in theupper layer 44U provided on a side of an upper surface of the electrode,and a range below the upper layer 44U is the lower layer 44L having atwo-phase configuration in which the pore CV is not present.

In other words, in the measurement electrode 44, the boundary (dashedline La) between the base part formed of the solid electrolyte (moreparticularly, the first solid electrolyte layer 4) and the measurementelectrode 44 is different from the boundary (dashed line Lb) between theregion where the pore CV is present and the region where the pore CV isnot present.

<Example of Identification of Locations of Boundaries>

A way to identify locations of the respective boundaries (dashed linesLa and Lb) when the locations of the boundaries are evaluated will bedescribed next with an example, which are a location of the boundary ofthe region where the noble metal NM is present and the region where thenoble metal NM is not present as the lowermost end of the range in whichthe noble metal NM is present and a location of the boundary of theregion where the pore CV is present and the region where the pore CV isnot present as the lowermost end of the range in which the pore CV ispresent in the thickness direction of the measurement electrode 44having a configuration as described above.

First, a cross-sectional image of the measurement electrode 44 along thethickness direction as shown in FIG. 2 is captured using a scanningelectron microscope (SEM) and the like, for example. In this case, animage capturing range is set so that the measurement electrode 44 is ashorizontal as possible in the captured image, and the captured imageincludes at least the entire range in the thickness direction of thelower layer 44L of the measurement electrode 44 and a portion near theboundary between the measurement electrode 44 and the first solidelectrolyte layer 4. Image capturing magnification is preferablyapproximately 500× to 1000× in view of the need to clearly identify eachof interfaces among the three phases in the captured image. In thiscase, the cross-sectional image in a range of approximately 100 μm to200 μm in the longitudinal direction of the element can be obtained.

Data of the obtained captured image is then analyzed based on a knownimage processing method to identify the ranges (all the pixelscorresponding to the ranges) in which the noble metal NM, the solidelectrolyte SE (and the first solid electrolyte layer 4), and the poreCV (and the third internal space 61) are present. In a case where thecaptured image is an SEM image, for example, which phases respectivepixels of the captured image correspond to can be identified by adifference in brightness.

A coordinate point (pixel location) providing a location of a lowermostend of the noble metal NM in the captured image is then identified basedon the data of the captured image. In a case of FIG. 2, a point Acorresponds to the coordinate point. A straight line passing through thepoint A and being parallel to a horizontal direction of the capturedimage is to correspond to the dashed line La showing the lowermost endof the range in which the noble metal NM is present.

However, the measurement electrode 44 might be inclined to no smallextent in the captured image, although the measurement electrode 44 isas horizontal as possible at image capturing. In light of the foregoing,the dashed line La showing the lowermost end of the range in which thenoble metal NM is present may be set by identifying, from a range of thenoble metal NM in the captured image, the coordinate point (e.g., thepoint A in FIG. 2) of the location of the lowermost end and a coordinatepoint (e.g., a point B in FIG. 2) at the second lowest location,correcting the captured image so that a straight line passing throughthe point A and the point B is horizontal, and determining the straightline passing through the point A and the point B in the correctedcaptured image as the dashed line La.

When the dashed line La is identified, a coordinate point (pixellocation) providing a location of a lowermost end of the pore CV in the(corrected) captured image is then identified based on the data of thecaptured image. In a case of FIG. 2, a point C corresponds to thecoordinate point. A straight line passing through the point C and beingparallel to the horizontal direction of the captured image is tocorrespond to the dashed line Lb showing the lowermost end of the rangein which the pore CV is present.

<Region Boundary Variation of Solid Electrolyte Region>

In the sensor element 101 according to the present embodiment, thecontiguous region RE as a portion of the solid electrolyte SE formingthe measurement electrode 44 is contiguous with the base part formed ofthe solid electrolyte as described above. The majority of the contiguousregion RE is the lower contiguous region REL belonging to the lowerlayer 44L in which the pore is not present, while a portion of thecontiguous region RE is the upper contiguous region REU furthercontiguous with the lower contiguous region REL and belonging to theupper layer 44U.

Since the pore CV is not present in the lower layer 44L, portions whichare in a lower end portion of the lower layer 44L and in which the solidelectrolyte SE is present constitute the contiguous region RE as awhole. The contiguous region RE is thus significantly formed in thesensor element 101 according to the present embodiment compared with aconventional sensor element in which the pore CV is present up to thelower end portion of the lower layer 44L.

Significant formation of the contiguous region RE in the measurementelectrode 44 means that a boundary (hereinafter, also referred to as aregion boundary) of a region formed of the solid electrolyte andcontiguous upward from a side of the first solid electrolyte layer 4 inthe thickness direction of the element (i.e., a region including thefirst solid electrolyte layer 4 and the contiguous region RE,hereinafter generically referred to as a first region) and a regionoccupied by the noble metal NM and the pore CV (hereinafter genericallyreferred to as a second region) is not along the boundary of the firstsolid electrolyte layer 4 and the measurement electrode 44 (i.e., dashedline La), and varies significantly in the thickness direction of theelement.

FIG. 3 is a diagram for describing a way to evaluate a degree ofvariation of the boundary of the first region and the second region inthe thickness direction of the element (hereinafter, region boundaryvariation degree) by taking, as an example, the model diagram showingthe partial cross section of the measurement electrode 44 shown in FIG.2.

As shown in FIG. 3, when a straight line Lc matching or being parallelto the dashed line La showing the lowermost end of the range in whichthe noble metal NM is present is set to an x-axis, and a straight lineis extended from any point represented by an equation x=x(i) on thex-axis as a starting point in the thickness direction of the elementperpendicular to the dashed line La or the straight line Lc, one of thelower end of the noble metal NM and the lower end of the pore CV thatthe straight line reaches first is set to a region boundary point on thestraight line. When the region boundary varies significantly in thethickness direction of the element, a length d(i) of a line segment(shown as an arrow in FIG. 3) extending from the starting point to theregion boundary tends to vary significantly with the location of theselected starting point. Since the pore CV is present only in the upperlayer 44U, a portion in which the region boundary point is anintersection of the pore CV and the first region can be considered as aportion in which the first region has entered into the second regionparticularly significantly.

In light of the foregoing, when many starting points represented by anequation x=x(1) to x(n) are set at predetermined intervals along thestraight line Lc, and region boundary points corresponding to therespective starting points are identified as shown in FIG. 3, thestandard deviation σ of lengths d(1) to d(n) of line segments extendingfrom the starting points to the respective region boundary points isindicative of the region boundary variation degree. That is to say, theboundary between the first region and the second region variessignificantly in the thickness direction of the element when thestandard deviation σ has a greater value, and the boundary between thefirst region and the second region is along the boundary (i.e., dashedline La) between the first solid electrolyte layer 4 and the measurementelectrode 44 when the standard deviation σ has a value closer to zero.

In identifying the distance d(i), the noble metal NM completelysurrounded by the solid electrolyte SE, and not in contact with the poreCV may be ignored. This is because the contiguous region RE is stillpresent above the noble metal NM in the thickness direction of theelement.

Although the straight line Lc is different from the dashed line La inFIG. 3 for convenience in description, they may match each other. Thisis because, although an average value of the distance d(i) varies withthe location of the straight line Lc, the standard deviation σ does notdepend on the location of the straight line Lc.

Although the starting points are set at discrete intervals in FIG. 3 forpurposes of illustration, the starting points are actually preferablyset at smaller intervals of 0.1 μm, for example.

FIG. 4 is a model diagram showing a partial cross section of ameasurement electrode 44Z having a configuration in which three phasesrandomly coexist in the electrode as a whole shown for comparison as atarget for evaluation of the region boundary variation degree. Alocation at which the measurement electrode 44Z is disposed in thesensor element is the same as a location at which the measurementelectrode 44 is disposed in the sensor element. A portion of white inFIG. 4 is the portion formed of the noble metal NM, a portion of gray inFIG. 4 is the portion formed of the solid electrolyte SE, and a portionof black in FIG. 4 is the portion formed of the pore CV also in themeasurement electrode 44Z shown in FIG. 4.

In the measurement electrode 44Z, the boundary between the region wherethe pore CV is present and the region where the pore CV is not presentgenerally matches the boundary between the base part formed of the solidelectrolyte (more particularly, the first solid electrolyte layer 4) andthe measurement electrode 44. That is to say, an appreciable portion ofthe base part is in contact with the pore CV or the noble metal NM.Although the contiguous region RE as a portion of the solid electrolyteSE contiguous with the base part is present, a degree of formationthereof is small as the pore CV is present.

Thus, in a case of FIG. 4, almost no difference can be seen among thelengths d(1) to d(n) of the line segments obtained as in a case of FIG.3. The region boundary variation degree of the sensor element includingthe measurement electrode 44Z thus has a smaller value than the regionboundary variation degree of the sensor element 101 including themeasurement electrode 44 having the two-layer configuration.

This suggests that the region boundary variation degree has a valuereflecting the configuration of the measurement electrode 44.

<Relationship between Configuration of Measurement Electrode andSuppression of Deterioration>

The measurement electrode 44 and the measurement electrode 44Z have amass transfer process in the electrode in common. That is to say, NOxand trace amounts of O₂, which are contained in the measurement gashaving reached the measurement electrode 44 or the measurement electrode44Z disposed in the third internal space 61 from outside the element,transfer through the pore CV, and come into contact with the noble metalNM as a catalyst. Pt as a main component of the noble metal NM causesdecomposition of NOx to generate O₂, and strips off an electron from O₂to generate an oxygen ion. The generated oxygen ion is taken, at atwo-phase interface of the noble metal NM and the solid electrolyte SE,from Pt into the solid electrolyte SE, and transfers within the sensorelement 101 due to a potential difference caused between the measurementelectrode 44 or the measurement electrode 44Z and the outer pumpelectrode 23 in the measurement pump cell 41.

In a case of the measurement electrode 44Z having a three-phaseconfiguration as a whole and thus having a small value of the regionboundary variation degree, however, the pore CV is present near oradjacent to the first solid electrolyte layer 4, and an interface of thesolid electrolyte SE and the pore CV is present at a relatively highratio, whereas an interface of the noble metal NM and the solidelectrolyte SE is present at a relatively low ratio. Thus, surplusoxygen coming into contact with the noble metal but not taken into thesolid electrolyte is likely to be generated, and thus deterioration ofthe measurement electrode caused by oxidation of the noble metalcontaining Pt as the main component by the surplus oxygen is likely tooccur.

In contrast, in the case of the measurement electrode 44 including thelower layer 44L having the two-phase configuration and the upper layer44U having the three-phase configuration, and thus having a large valueof the region boundary variation degree, while the pore CV through whichoxygen is distributed is present only in the upper layer 44U, the solidelectrolyte SE present in the lower layer 44L is almost in the lowercontiguous region REL contiguous with the base part, and is only incontact with the noble metal NM. Thus, the two-phase interface of thenoble metal NM and the solid electrolyte SE is present at a higher ratiothan that in a conventional gas sensor, and, in particular, only thetwo-phase interface is present in the lower layer 44L, so that thesurplus oxygen is less likely to accumulate in the pore CV to be incontact with the noble metal NM, and deteriorate the measurementelectrode 44.

That is to say, in the gas sensor 100 according to the presentembodiment, deterioration of the measurement electrode 44 due to thepresence of the surplus oxygen is suitably suppressed. Deterioration ofmeasurement sensitivity is thus suitably suppressed even if the gassensor 100 is used continuously.

The region boundary variation degree is preferably 0.5 μm or more and6.5 μm or less.

In this case, the rate of change of the NOx current Ip2 with respect toan initial value thereof when the gas sensor 100 is used continuously issuppressed to 6% or less. That is to say, a rate of deterioration ofmeasurement sensitivity when the gas sensor 100 is used continuously issuppressed to 6% or less.

The region boundary variation degree is more preferably 1 μm or more and5 μm or less. In this case, the rate of change of the NOx current Ip2with respect to the initial value thereof when the gas sensor 100 isused continuously, that is, the rate of deterioration of measurementsensitivity when the gas sensor 100 is used continuously is suppressedto approximately 3% or less. Furthermore, reduction in pumping capacityof the measurement pump cell 41 due to insufficiency of the number ofsites for ionization of the measurement gas, which can be caused when avalue of the region boundary variation degree is large and thereforethere are few pores in the measurement electrode 44, is suppressed.

Furthermore, it is preferable that the measurement electrode 44 have athickness of 10 μm to 30 μm, and a thickness ratio t_(U):t_(L) be in arange of 95:5 to 10:90. In other words, a ratio of a distance to from anuppermost portion of the measurement electrode 44 to the lowermost end(dashed line Lb) of the range in which the pore CV is present withrespect to a distance t_(L) from a lowermost portion of the measurementelectrode 44 to the lowermost end of the range in which the pore CV ispresent is in the range of 95:5 to 10:90.

A volume ratio of the solid electrolyte SE in the upper layer 44U is 20%to 40%, and a volume ratio of the solid electrolyte SE in the lowerlayer 44L is 50% to 60%.

In addition, the configuration of the measurement electrode 44 in whichthe solid electrolyte contiguous with the first solid electrolyte layer4 enters into a noble metal portion and which has a large value of theregion boundary variation degree produces the so-called anchoringeffect. The configuration of the measurement electrode 44 used in thegas sensor 100 according to the present embodiment is thus effective atsuppressing separation of the measurement electrode 44.

<Process of Manufacturing Sensor Element>

A process of manufacturing the sensor element 101 having a configurationand features as described above will be described next. In the presentembodiment, a laminated body including green sheets (also referred to asbase material tapes) containing zirconia as a ceramic component isformed, and the laminated body is cut and fired to manufacture thesensor element 101.

Description will be made below by taking, as an example, a case wherethe sensor element 101 including the six layers shown in FIG. 1 ismanufactured. In this case, six green sheets corresponding to the firstsubstrate layer 1, the second substrate layer 2, the third substratelayer 3, the first solid electrolyte layer 4, the spacer layer 5, andthe second solid electrolyte layer 6 are to be prepared. FIG. 5 is aflowchart of processing at the manufacture of the sensor element 101.

In a case where the sensor element 101 is manufactured, blank sheets(not illustrated) being green sheets having no pattern formed thereonare prepared first (step S1). In a case where the sensor element 101including the six layers is manufactured, six blank sheets correspondingto the respective layers are prepared.

The blank sheets have a plurality of sheet holes used for positioning inprinting and lamination. The sheet holes are formed to the blank sheetsin advance prior to pattern formation through, for example, punching bya punching machine. Green sheets corresponding to layers in which aninternal space is formed also include penetrating portions correspondingto the internal space formed in advance through, for example, punchingas described above. The blank sheets corresponding to the respectivelayers of the sensor element 101 are not required to have the samethickness.

After preparation of the blank sheets corresponding to the respectivelayers, pattern printing and drying are performed on the individualblank sheets (step S2). Specifically, a pattern of various electrodes, apattern of the heater element 72 and the heater insulating layer 74, apattern of internal wiring, which is not illustrated, and the like areformed.

Application or placement of a sublimable material to form the firstdiffusion control part 11, the second diffusion control part 13, and thethird diffusion control part 30 is also performed at the time of patternprinting.

The patterns are printed by applying pastes for pattern formationprepared in accordance with the properties required for respectiveformation targets onto the blank sheets using known screen printingtechnology. A known drying means can be used for drying after printing.

A method different from a conventional method is used to form, fromamong these patterns, a pattern to eventually be the measurementelectrode 44. In this regard, description will be made below.

After pattern printing on each of the blank sheets, printing and dryingof a bonding paste are performed to laminate and bond the green sheetscorresponding to the respective layers (step S3). The known screenprinting technology can be used for printing of the bonding paste, andthe known drying means can be used for drying after printing.

The green sheets to which an adhesive has been applied are then stackedin a predetermined order, and the stacked green sheets are crimped underpredetermined temperature and pressure conditions to thereby form alaminated body (step S4). Specifically, crimping is performed bystacking and holding the green sheets as a target of lamination on apredetermined lamination jig, which is not illustrated, whilepositioning the green sheets at the sheet holes, and then heating andpressurizing the green sheets together with the lamination jig using alamination machine, such as a known hydraulic pressing machine. Thepressure, temperature, and time for heating and pressurizing depend on alamination machine to be used, and these conditions may be determinedappropriately to achieve good lamination.

After the laminated body is obtained as described above, the laminatedbody is cut at a plurality of locations into individual units (referredto as element bodies) of the sensor elements 101 (step S5).

The element bodies obtained by cutting are each fired at a firingtemperature of approximately 1300° C. to 1500° C. (step S6). The sensorelement 101 is thereby manufactured. That is to say, the sensor element101 is generated by integrally firing the solid electrolyte layers andthe electrodes. The firing temperature in this case is preferably 1200°C. or more and 1500° C. or less (e.g., 1400° C.). Integral firing isperformed in this manner, so that the electrodes each have sufficientadhesion strength in the sensor element 101.

The sensor element 101 thus obtained is housed in a predeterminedhousing, and built into the body (not illustrated) of the gas sensor100.

<Method for Forming Measurement Electrode>

As described above, in the present embodiment, the measurement electrode44 having a two-layer configuration as described above is provided inthe sensor element 101. Formation of the measurement electrode 44 willbe described.

FIG. 6 is a diagram showing procedures for forming a pattern toeventually be the measurement electrode 44. There are two methods forforming the pattern to be the measurement electrode 44: a manufacturingmethod A, and a manufacturing method B.

In the manufacturing method A, an electrolyte paste containing, as aceramic component, zirconia as with the green sheets is first applied ata location, on an upper surface of a green sheet to form the first solidelectrolyte layer 4, as a target of formation of the measurementelectrode 44 (step S21A). The electrolyte paste is a paste obtained bymixing powder of zirconia as the solid electrolyte and an organiccomponent including a binder and the like.

Then, a three-component mixed paste is superimposedly applied onto anapplied film formed by application of the electrolyte paste (step S22).The three-component mixed paste is herein a paste obtained by mixingpowder of the noble metal containing Pt as the main component, powder ofzirconia as the ceramic component, powder of a pore forming material asa sublimable material to form the pore CV in the measurement electrode44 eventually formed, and an organic component including a binder andthe like.

When application of the three-component mixed paste is completed, apaste applied film (paste-doubled film) where the electrolyte paste andthe three-component mixed paste are superimposed is pressed by apredetermined pressing means (step S23). By pressing, some of particlesof the noble metal and the pore forming material included in the appliedfilm of the three-component mixed paste enter into the applied film ofthe electrolyte paste.

After that, when firing targeted for an element body is performed asdescribed above, the paste-doubled film is also fired, andvolatilization of the organic component and, further, sintering of thenoble metal NM and the solid electrolyte SE progress. In this case, thesolid electrolyte in the electrolyte paste is integrated with the solidelectrolyte in the green sheet, and a portion in which some particles ofthe noble metal have entered into the electrolyte paste becomes thelower layer 44L with the progress of sintering. On the other hand, in aportion in which the three-component mixed paste is applied, the poreforming material sublimates with the progress of sintering of the noblemetal NM and the solid electrolyte SE to form the pore CV, and the upperlayer 44U in which the noble metal NM, the solid electrolyte SE, and thepore CV coexist is eventually obtained. As a result, the measurementelectrode 44 having a two-layer configuration as shown in FIG. 2 isformed.

Application thicknesses and application areas of the electrolyte pasteand the three-component mixed paste are only required to be set bytaking into account contraction by firing and entry of the particles ofthe noble metal and the pore forming material by pressing so that thethicknesses of the upper layer 44U and the lower layer 44L of themeasurement electrode 44 eventually formed and a planar area of themeasurement electrode 44 have desired values. For example, arelationship between the application thicknesses and the applicationareas of the electrolyte paste and the three-component mixed paste andthe thicknesses of the upper layer 44U and the lower layer 44L of themeasurement electrode 44 and the planar area of the measurementelectrode 44 may experimentally be identified in advance. A mixing ratioof the powder of the noble metal, the powder of zirconia as the solidelectrolyte, and the powder of the pore forming material of thethree-component mixed paste is only required to be set by taking intoaccount entry of the particles of the noble metal and the pore formingmaterial by pressing so that the volume ratios of the noble metal NM,the solid electrolyte SE, and the pore CV in the upper layer 44U(three-phase region) of the measurement electrode 44 eventually formedhave desired values.

On the other hand, in the manufacturing method B, a two-component mixedpaste is applied at the location, on the upper surface of the greensheet to form the first solid electrolyte layer 4, as the target offormation of the measurement electrode 44 (step S21B). The two-componentmixed paste is herein a paste obtained by mixing the powder of the noblemetal containing Pt as the main component, the powder of zirconia as theceramic component, and an organic component including a binder and thelike.

Hereinafter, the three-component mixed paste is superimposedly appliedonto an applied film formed by application of the two-component mixedpaste (step S22), and, further, a paste applied film (paste-doubledfilm) where the two-component mixed paste and the three-component mixedpaste are superimposed is pressed by a predetermined pressing means(step S23), as in the manufacturing method A.

Also in a case of the manufacturing method B, some of particles of thenoble metal and the pore forming material included in the coating of thethree-component mixed paste enter into the coating of the two-componentmixed paste by pressing.

After that, when firing targeted for the element body is performed asdescribed above, the paste-doubled film is also fired, andvolatilization of the organic component and, further, sintering of thenoble metal NM and the solid electrolyte SE progress. A portion in whichthe two-component mixed paste is applied thus becomes the lower layer.On the other hand, in the portion in which the three-component mixedpaste is applied, the pore forming material sublimates with the progressof sintering of the noble metal NM and the solid electrolyte SE to formthe pore CV, and the upper layer 44U in which the noble metal NM, thesolid electrolyte SE, and the pore CV coexist is eventually obtained.Also in this case, as a result, the measurement electrode 44 having atwo-layer configuration as shown in FIG. 2 is formed.

Also in the case of the manufacturing method B, the applicationthicknesses and the application areas of the two-component mixed pasteand the three-component mixed paste are only required to be set bytaking into account contraction by firing and entry of the particles ofthe noble metal and the pore forming material by pressing so that thethicknesses of the upper layer 44U and the lower layer 44L of themeasurement electrode 44 eventually formed and the planar area of themeasurement electrode 44 have desired values. A mixing ratio of thepowder of the noble metal and the powder of zirconia as the solidelectrolyte of the two-component mixed paste is only required to be setby taking into account entry of the particles of the noble metal and thepore forming material by pressing so that the volume ratios of the noblemetal NM and the solid electrolyte SE in the lower layer 44L (two-phaseregion) of the measurement electrode 44 eventually formed have desiredvalues. Similarly, the mixing ratio of the powder of the noble metal,the powder of zirconia as the solid electrolyte, and the powder of thepore forming material of the three-component mixed paste is onlyrequired to be set by taking into account entry of the particles of thenoble metal and the pore forming material by pressing so that the volumeratios of the noble metal NM, the solid electrolyte SE, and the pore CVin the upper layer 44U (three-phase region) of the measurement electrode44 eventually formed have desired values. As in the case of themanufacturing method A, the relationship among these values mayexperimentally be identified in advance.

As described above, according to the present embodiment, the measurementelectrode of the sensor element of the limiting current gas sensor isprovided to have the two-layer configuration despite that it is providedas the porous cermet electrode as a whole, including the lower layer inwhich only the noble metal and the solid electrolyte are present and thepore is not present and the upper layer in which the noble metal, thesolid electrolyte, and the pore coexist, and thereby the region boundaryvariation degree as for the region formed of the solid electrolyte andthe region occupied by the noble metal and the pore has a large value,so that deterioration of the measurement electrode due to the presenceof the surplus oxygen not taken into the measurement electrode issuitably suppressed. A gas sensor in which deterioration of measurementsensitivity is suitably suppressed even if the gas sensor is usedcontinuously is thereby achieved.

<Modifications>

A configuration used in the measurement electrode 44 in theabove-mentioned embodiment in which the measurement electrode 44 has thetwo-layer configuration including the region where the pore is notpresent and the region where the pore is present, and thereby the regionboundary variation degree as for the region formed of the solidelectrolyte and the region occupied by the noble metal and the pore hasa large value may be applied to the inner pump electrode 22 and theauxiliary pump electrode 51 each provided as a cermet of the Au—Pt alloyand ZrO₂ as with the measurement electrode 44. In this case, aseparation suppressive effect associated with the anchoring effect canalso be obtained in these electrodes. The inner pump electrode 22 andthe auxiliary pump electrode 51 having such a configuration can beformed by a similar process to the measurement electrode 44.

Although the two methods, that is, the manufacturing method A and themanufacturing method B, are shown in the above-mentioned embodiment asthe method for forming the pattern to be the measurement electrode 44, amanufacturing method C may be used in place of these methods.

In the manufacturing method C, the electrolyte paste used in themanufacturing method A, the two-component mixed paste used in themanufacturing method B, and the three-component mixed paste used in themanufacturing method A and the manufacturing method B are applied in thestated order at the location, on the upper surface of the green sheet toform the first solid electrolyte layer 4, as the target of formation ofthe measurement electrode 44, and then are pressed by a predeterminedpressing means.

Also in a case of the manufacturing method C, the applicationthicknesses and the application areas of the electrolyte paste, thetwo-component mixed paste, and the three-component mixed paste are onlyrequired to be set by taking into account contraction by firing andentry of the particles of the noble metal and the pore forming materialby pressing so that the thicknesses of the upper layer 44U and the lowerlayer 44L of the measurement electrode 44 eventually formed and theplanar area of the measurement electrode 44 have desired values. Themixing ratio of the powder of the noble metal and the powder of zirconiaas the solid electrolyte of the two-component mixed paste is onlyrequired to be set by taking into account entry of the particles of thenoble metal and the pore forming material by pressing so that the volumeratios of the noble metal NM and the solid electrolyte SE in the lowerlayer 44L (two-phase region) of the measurement electrode 44 eventuallyformed have desired values. Similarly, the mixing ratio of the powder ofthe noble metal, the powder of zirconia as the solid electrolyte, andthe powder of the pore forming material of the three-component mixedpaste is only required to be set by taking into account entry of theparticles of the noble metal and the pore forming material by pressingso that the volume ratios of the noble metal NM, the solid electrolyteSE, and the pore CV in the upper layer 44U (three-phase region) of themeasurement electrode 44 eventually formed have desired values. As inthe case of the manufacturing method A and the manufacturing method B,the relationship among these values may experimentally be identified inadvance.

EXAMPLES

Six types of gas sensors 100 (Examples 1 to 6) differing in conditionsfor manufacturing the measurement electrode 44 were manufactured asexamples, and, for each of the obtained gas sensors 100, volume ratiosof respective phases (the noble metal, the solid electrolyte, and thepore) in each of the upper layer 44U and the lower layer 44L of themeasurement electrode 44 and the region boundary variation degree wereevaluated. A continuous drive test in air was also conducted to evaluatea degree of deterioration of the measurement electrode with continueduse.

At formation of the measurement electrodes 44, the measurementelectrodes 44 were manufactured by two different manufacturing methods,that is, the manufacturing method A and the manufacturing method B, and,for each of the manufacturing methods, the three-component mixed pastesto mainly form the upper layers 44U had two different weight ratios ofthe powder of the noble metal, the powder of the solid electrolyte, andthe pore forming material. One type of the electrolyte paste was used inthe manufacturing method A, and one type of the two-component mixedpaste was used in the manufacturing method B.

A gas sensor including the measurement electrode 44Z formed using thethree-component mixed paste as a whole was also manufactured as aconventional example (a method for manufacturing the gas sensor ishereinafter referred to as a conventional manufacturing method), and,for the manufactured gas sensor, the volume ratios of the respectivephases (the noble metal, the solid electrolyte, and the pore) of themeasurement electrode 44Z were evaluated, and the continuous drive testin air was conducted as with the gas sensors of the examples.

Weight ratios (component ratios) of the powder of the noble metal, thepowder of the solid electrolyte, and the pore forming material of thepastes used to manufacture the measurement electrodes 44 of Examples 1to 6 and the measurement electrode 44Z of the conventional example areshown in Table 1 as a list.

TABLE 1 MEASUREMENT ELECTRODE PASTE COMPONENT RATIOS (WEIGHT RATIOS)MEASUREMENT FOR UPPER LAYER FORMATION (a:b:c) FOR LOWER LAYER FORMATION(d:e) ELECTRODE NOBLE PORE FORMING NOBLE FORMATION METAL ELECTROLYTEMATERIAL METAL ELECTROLYTE METHOD a b c USED PASTE d e CONVENTIONALCONVENTIONAL 10 2 1 NOT USED — — EXAMPLE MANUFACTURING METHOD EXAMPLE 1MANUFACTURING 10 2 1 ELECTROLYTE — 10 METHOD A PASTE EXAMPLE 2MANUFACTURING 30 10 1 — 10 METHOD A EXAMPLE 3 MANUFACTURING 30 10 1 — 10METHOD A EXAMPLE 4 MANUFACTURING 10 2 1 TWO- 5 1 METHOD B COMPONENTEXAMPLE 5 MANUFACTURING 30 10 1 MIXED PASTE 5 1 METHOD B EXAMPLE 6MANUFACTURING 10 2 1 5 1 METHOD B

In Table 1, weight ratios of the three-component mixed pastes are shownin columns “FOR UPPER LAYER FORMATION”, and weight ratios of theelectrolyte pastes and weight ratios of the two-component mixed pastesare shown in columns “FOR LOWER LAYER FORMATION”.

When a, b, and c are respectively weight ratios of the powder of thenoble metal, the powder of the solid electrolyte, and the pore formingmaterial of the three-component mixed pastes, and d and e arerespectively weight ratios of the powder of the noble metal and thepowder of the solid electrolyte of the two-component mixed pastes, theweight ratios of the three-component mixed pastes satisfy an equationa:b:c=10:2:1 (Examples 1, 4, and 6) or an equation a:b:c=30:10:1(Examples 2, 3, and 5) as shown in Table 1. When the mixing ratio c ofthe pore forming material of each of the three-component mixed pastes isone, the two-component mixed pastes satisfy an equation (c:)d:e=(1:)5:1(Examples 4, 5, and 6). Furthermore, as for the weight ratio of thepowder of the solid electrolyte of each of the electrolyte pastes, whichis expressed as e for the sake of convenience, relative to the mixingratio c of the pore forming material of each of the three-componentmixed pastes, an equation c:e=1:10 (Examples 1 and 2) is satisfied.

In each of Examples 1 to 3 in which the measurement electrode 44 isformed by the manufacturing method A, an intended application thicknessof the electrolyte paste is 10 μm, and an intended application thicknessof the three-component mixed paste is 15 μm.

In each of Examples 4 to 6 in which the measurement electrode 44 isformed by the manufacturing method B, an intended application thicknessof the two-component mixed paste is 5 μm, and the intended applicationthickness of the three-component mixed paste is 15 μm.

In the conventional example in which the measurement electrode 44Z ismanufactured by the conventional manufacturing method, thethree-component mixed paste satisfies the equation a:b:c=10:2:1, and theintended application thickness of the three-component mixed paste is 15μm.

The volume ratios of the respective phases in each of the upper layer44U and the lower layer 44L of the measurement electrode 44 and resultsof evaluation of the region boundary variation degree in each of theexamples and the conventional example are shown in Table 2. The volumeratios in the measurement electrode 44Z of the conventional example areshown in a column “UPPER LAYER” for the sake of convenience.

TABLE 2 ELECTROLYTE MEASUREMENT ELECTRODE VOLUME RATIOS (%): BOUNDARYLOCATION IN RELATION TO EACH LAYER AS A WHOLE AS 100% VARIATION UPPERLAYER (THREE-PHASE REGION) LOWER LAYER (TWO-PHASE REGION) AVERAGESTANDARD NOBLE METAL ELECTROLYTE PORE NOBLE METAL ELECTROLYTE VALUEDEVIATION P1 P2 P3 P4 P5 (μm) σ (μm) CONVENTIONAL 40 20 40 — — 0 0EXAMPLE EXAMPLE 1 40 20 40 40 60 1.0 1.1 EXAMPLE 2 45 40 15 45 55 2.11.9 EXAMPLE 3 45 40 15 45 55 0.4 0.5 EXAMPLE 4 40 20 40 40 60 4.9 5.0EXAMPLE 5 45 40 15 40 60 2.5 2.6 EXAMPLE 6 40 20 40 40 60 6.1 6.3

In Table 2, P1 (%), P2 (%), and P3 (%) are respectively the volumeratios of the noble metal NM, the solid electrolyte SE, and the pore CVin the upper layer 44U, and P4 (%) and P5 (%) are respectively thevolume ratios of the noble metal NM and the solid electrolyte SE in thelower layer 44L. An equation P1+P2+P3=P4+P5=100 (%) is satisfied.

The volume ratios of the respective phases in each of the measurementelectrodes 44 and the measurement electrode 44Z were obtained bycapturing a cross-sectional SEM image of the electrode, and performingknown image processing on the cross-sectional SEM image. Thecross-sectional SEM image was generally ternary valued so that theregion where the noble metal NM was present was white, the region wherethe solid electrolyte SE was present was gray, and the region where thepore or the internal space was present was black, and ratios of theareas (ratios of the cross-sectional areas) of the respective regionswere set to the volume ratios. The region boundary variation degree wasevaluated by measuring, from the SEM image of the cross sectionincluding the first solid electrolyte layer 4 and the measurementelectrode 44 and perpendicular to the thickness direction of the sensorelement, distances from the boundary of the first region and the secondregion to the lowermost end of the range in which the noble metal NM waspresent at intervals of 0.1 μm in the longitudinal direction of theelement, and determining the average value and the standard deviation athereof. A value of the standard deviation a corresponds to the regionboundary variation degree. The average value and the standard deviationa in the conventional example were both set to zero.

The results shown in Table 2 indicate that the measurement electrode 44has the two-layer configuration including the upper layer 44U and thelower layer 44L in each of Examples 1 to 6.

More particularly, values of P1, P2, P3, P4, and P5 are the same inExamples 1, 4, and 6, which differ in the method for manufacturing themeasurement electrode 44, but have in common that the weight ratios ofthe three-component mixed pastes satisfy the equation a:b:c=10:2:1. Fromamong these values, the volume ratio of the noble metal NM in each ofthe upper layer 44U and the lower layer 44L satisfies an equationP1=P4=40%. As for the volume ratio of the solid electrolyte SE, thevalue of P2 in the upper layer 44U is 20%, whereas the value of P5 inthe lower layer 44L is 60%, which is greater than the value of P2.

In contrast, in Examples 2, 3, and 5, which differ in the method formanufacturing the measurement electrode 44, but have in common that theweight ratios of the three-component mixed pastes satisfy the equationa:b:c=30:10:1, the values of P1, P2, and P3 are the same, but the valuesof P4 and P5 in Examples 2 and 3 differ from those in Example 5 by 5%.That is to say, the volume ratio P1 of the noble metal NM in the upperlayer 44U has a value of 45%, and the volume ratio P2 of the solidelectrolyte SE in the upper layer 44U has a value of 40% in each ofExamples 2, 3, and 5, but the volume ratio P4 of the noble metal NM inthe lower layer 44L has a value of 45% in each of Examples 2 and 3, andhas a value of 40% in Example 5. In response to these values, the volumeratio P5 of the solid electrolyte SE has a value of 55% in each ofExamples 2 and 3, and has a value of 60% in Example 5.

In each of the examples, however, the volume ratio of the solidelectrolyte SE in the lower layer 44L is greater than the volume ratioof the solid electrolyte SE in the upper layer 44U. Specifically, thevolume ratio of the solid electrolyte SE in the upper layer 44U is in arange of 20% to 40%, and the volume ratio of the solid electrolyte SE inthe lower layer 44L is in a range of 50% to 60%.

The volume ratios P1, P2, and P3 of the respective phases in themeasurement electrode 44Z of the conventional example are the same asthose of Examples 1 and 4 in each of which the three-component mixedpaste having the same weight ratios is used.

The standard deviation σ corresponding to the region boundary variationdegree has a greater value in each of Examples 4 to 6 manufactured bythe manufacturing method B than in each of Examples 1 to 3 manufacturedby the manufacturing method A. Examples (Examples 2 and 3, and Examples4 and 6) having the manufacturing method and the weight ratios in thethree-component mixed paste in common, however, differ in value of thestandard deviation σ, and a correspondence relationship between theweight ratios and the value of the standard deviation σ varies dependingon the manufacturing method.

The continuous drive test in air was conducted by continuously operatingeach of the gas sensors in an air atmosphere at 25±5° C. for 3000 hours,and measuring values of the NOx current (also referred to as NOx output)at the start of operation, after the elapse of 1000 hours, after theelapse of 2000 hours, and after the elapse of 3000 hours. The rate ofchange of the NOx current at each measurement time to a value of the NOxcurrent at the start of operation was calculated as a NOx output changerate. The NOx output change rate is a value indicative of a degree ofdeterioration of each of the measurement electrodes 44 and themeasurement electrode 44Z.

FIG. 7 is a graph showing results of the continuous drive test. In FIG.7, a horizontal axis represents a drive time, and a vertical axisrepresents the NOx output change rate.

As shown in FIG. 7, in the gas sensor of the conventional example, theNOx output significantly changes over the drive time, and the NOx outputchange rate taking into account variation is approximately 11.5±3.5%after the elapse of 3000 hours, whereas, in the gas sensor of each ofExamples 1 to 6, the NOx output change rate remains at most atapproximately 6% even after the elapse of 3000 hours. This means thatdeterioration of the measurement electrode is suppressed even after theelapse of 3000 hours in the gas sensor of each of Examples 1 to 6.

In particular, in the gas sensor of each of examples other than Example3, the NOx output change rate generally remains at 3% or less after theelapse of 3000 hours, and it is determined that deterioration of themeasurement electrode is more suitably suppressed in the gas sensor.

The above-mentioned results of the continuous drive test indicate that,when the measurement electrode of the sensor element of the limitingcurrent type gas sensor is provided to have the two-layer configurationincluding the lower layer having the two-phase configuration of thenoble metal and the solid electrolyte and the upper layer having thethree-phase configuration of the noble metal, the solid electrolyte, andthe pore, and thereby the region boundary variation degree has a largevalue, deterioration of the measurement electrode when the gas sensor isused continuously can be suppressed.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A sensor element of a limiting current type NOxsensor, the sensor element comprising: a base part containing anoxygen-ion conductive solid electrolyte as a constituent material; atleast one internal space into which a measurement gas is introduced; andat least one pump cell including an internal electrode disposed to facethe at least one internal space, an out-of-space pump electrode disposedat a location other than the at least one internal space, and a portionof the base part located between the internal electrode and theout-of-space pump electrode, wherein the internal electrode includes anoble metal, the solid electrolyte, and a pore, and in the internalelectrode, a region boundary variation degree being a degree ofvariation of a boundary of a first region and a second region in athickness direction of the element is 0.5 μm or more and 6.5 μm or less,the first region being formed of the base part or the solid electrolytecontiguous with the base part, the second region being occupied by thenoble metal and the pore.
 2. The sensor element of the NOx sensoraccording to claim 1, wherein the at least one internal space is ameasurement internal space into which a measurement gas having an oxygenconcentration adjusted in advance is introduced, the internal electrodeis a measurement electrode disposed in the measurement internal space,and the at least one pump cell is a measurement pump cell including themeasurement electrode, the out-of-space pump electrode, and a portion ofthe base part located between the measurement electrode and theout-of-space pump electrode.
 3. The sensor element of the NOx sensoraccording to claim 2, wherein the measurement electrode includes: anupper layer consisting of the noble metal, the solid electrolyte, andthe pore; and a lower layer consisting of the noble metal and the solidelectrolyte, and a volume ratio of the solid electrolyte in the upperlayer is 20% to 40%, and a volume ratio of the solid electrolyte in thelower layer is 50% to 60%.
 4. The sensor element of the NOx sensoraccording to claim 3, wherein a ratio of a thickness of the upper layerto a thickness of the lower layer is in a range of 95:5 to 10:90.
 5. Thesensor element of the NOx sensor according to claim 4, wherein the ratioof the thickness of the upper layer to the thickness of the lower layeris in a range of 90:10 to 30:70.
 6. The sensor element of the NOx sensoraccording to claim 1, wherein the region boundary variation degree is 1μm or more and 5 μm or less.
 7. The sensor element of the NOx sensoraccording to claim 2, wherein the region boundary variation degree is 1μm or more and 5 μm or less.
 8. The sensor element of the NOx sensoraccording to claim 3, wherein the region boundary variation degree is 1μm or more and 5 μm or less.
 9. The sensor element of the NOx sensoraccording to claim 4, wherein the region boundary variation degree is 1μm or more and 5 μm or less.
 10. The sensor element of the NOx sensoraccording to claim 5, wherein the region boundary variation degree is 1μm or more and 5 μm or less.